weapons.yml

removemultiplebows: true   #If this is enabled you will only have 1 bow a time
chest:
- stonesword
- woodsword
- bow
enderchest:
- ironsword
- void
stonesword:
   items: 
   - "STONE_SWORD:&lStone Sword"
   class: 1
   priority: 1
   chance: 1
woodsword:
   items: 
   - "WOOD_SWORD:&lWooden Sword"
   class: 1
   priority: 0
   chance: 3
bow:
   items:
   - "BOW:&lBow"
   - "ARROW:&lArrow:32"
   class: 2
   chance: 2
ironsword:
   items: 
   - "IRON_SWORD:&lIron Sword"
   class: 1
   priority: 2
   chance: 3   
void:
   items: 
   - "AIR:null"
   class: 3
   priority: 0
   chance: 1 

Explanation

Items of one class (in this example the swords) will replace each other. But they only replace each other when the priority of the Item is higher than the priority of the item you already have. In this example , you can get a Iron Sword when you already have a Stone Sword or a Wooden Sword but you cant get a Stone Sword or a Wood Sword if you already have an Iron Sword. If the priority of an item is set to 0 you can get unlimited of these items.

Chances:

(Example) If the chance of your Stone Sword is 1 and the chance of your Wood Sword is 4 , it will be 4 times more likely to get the Wood Sword.